DESCRIPTION: There are currently no treatments capable of restoring cardiac function after myocardial infarction (MI) besides cardiac transplantation. MI kills millions of heart cells due to reduced oxygen and blood supply. Strategies to regenerate damaged heart cells that use proteins, such as neuregulin (NRG1), to stimulate mitosis of surviving cardiomyocytes could partially restore infarcted myocardium. However, delivering such therapeutics to the heart using conventional methods is difficult for two reasons: (1) heart blood vessels show very low permeability for large molecules, (2) molecules that do reach the heart are washed away rapidly by the high blood flow. In clinical studies, NRG1's short half-life necessitates daily systemic injections. Further, the fraction that reaches the heart is very low, compromising its therapeutic efficacy. This non-targeted approach can also lead to the uncontrolled proliferation of cardiomyocytes in the remote zone of the heart, causing myocardial hyperplasia and hypertrophy. Clearly there is an urgent need for a non-invasive and controlled delivery approach that can specifically target surviving cardiomyocytes in the infarcted area and border zone. We propose to develop a novel delivery strategy that fulfills this need. Our approach will target infarcted cardiac tissue by exploiting the body's immunological response to MI. Specifically, we propose to deliver regenerating proteins using monocytes that naturally migrate to the site of infarction. Monocytes show extraordinary retention in the heart in spite of high blood flow due to specific receptor-ligand interactions with the extracellular matrix and other proteins. We hypothesize that (1) monocytes can be targeted and genetically programmed with self-replicating RNA-nanoplexes to express NRG1 and that (2) monocytes will home to infarcted tissue and locally release NRG1, a protein that can induce cardiomyocyte proliferation and facilitate tissue regeneration. To test these hypotheses, we will first generate self-replicating RNA-nanoplexes that target monocytes and program them to express NRG1. Secondly, we will quantify the number of MI-homing monocytes that are genetically programmed for protein production in a mouse model of MI. We anticipate that these studies will lead to a living drug reservoir that is non-invasive and able to locally deliver protein therapeutics to the infarcted sie and its border zone. This has profound implications for the treatment of patients with IHD and the regeneration of infarcted myocardium.